1. Field of the Invention
This invention relates to gas sensors. More specifically, this invention relates to method and apparatus for detecting methane in a gas mixture also containing known interference gases.
2. Description of the Prior Art
Sensors for the detection of methane in air are known. Currently available methane sensors typically utilize semiconducting metal oxides. Also, reaction heat sensors or heat tone sensors called pellistors are also used to detect methane.
The metal oxide most often used as a sensor material is tin oxide, which is a semiconductor at higher temperatures. A tin oxide sensor that has been doped with platinum will react to the presence of methane and other reducing gases. The operating temperature of a Pt-doped SnO.sub.2 sensor is typically low, around 350.degree. C.
Another methane sensor known in the art has a thin film of Ga.sub.2 O.sub.3 as a gas-sensitive element. At temperatures greater than 700.degree. C., a Ga.sub.2 O.sub.3 sensor will generate a very strong measurement signal even when the methane concentration is well below the lower explosive limit of 5%. The Ga.sub.2 O.sub.3 sensor does not react to the presence of water vapor in air. However, like all known SnO.sub.2 sensors and pellistors, the Ga.sub.2 O.sub.3 sensors are extremely sensitive to the presence of alcohols. The reaction of a Ga.sub.2 O.sub.3 sensor to the presence of alcohol--a commonly used solvent--is called cross-sensitivity, and in some circumstances, will completely mask the reaction of the Ga.sub.2 O.sub.3 sensor to methane.
Therefore, a need exists for an improved methane gas sensor which is useful at high temperatures and in the presence of commonly used solvent vapors.